This invention relates to a system for measuring the properties of thin films, and more particularly to a system which optically induces stress pulses in a film and which optically measures the stress pulses propagating within the film.
Presently, the nondestructive evaluation of thin films and interfaces is of interest to manufacturers of electrical, optical and mechanical devices which employ thin films In one nondestructive technique a radio frequency pulse is applied to a piezoelectric transducer mounted on a substrate between the transducer and the film to be studied. A stress pulse propagates through the substrate toward the film. At the boundary between the substrate and the film, part of the pulse is reflected back to the transducer. The remainder enters the film and is partially reflected at the opposite side to return through the substrate to th e transducer. The pulses are converted into electrical signals, amplified electronically, and displayed on an oscilloscope. The time delay between the two pulses indicates the film thickness, if the sound velocity in the film is known, or indicates the sound velocity, if the film thickness is known. Relative amplitudes of the pulses provide information on the attenuation in the film or the quality of the bond between the film and the substrate.
The minimum thickness of films which can be measured and the sensitivity to film interface conditions using conventional ultrasonics is limited by the pulse length. The duration of the stress pulse is normally at least 0.1 xcexcsec corresponding to a spatial length of at least 3xc3x9710xe2x88x922 cm for an acoustic velocity of 3xc3x97105 cm/sec. Unless the film is thicker than the length of the acoustic pulse, the pulses returning to the transducer will overlap in time. Even if pulses as short in duration as 0.001 xcexcsec are used, the film thickness must be at least a few microns.
Another technique, acoustic microscopy, projects sound through a rod having a spherical lens at its tip. The tip is immersed in a liquid covering the film. Sound propagates through the liquid, reflects off the surface of the sample, and returns through the rod to the transducer. The amplitude of the signal returning to the transducer is measured while the sample is moved horizontally. The amplitudes are converted to a computer-generated photograph of the sample surface. Sample features below the surface are observed by raising the sample to bring the focal point beneath the surface. The lateral and vertical resolution of the acoustic microscope are approximately equal.
Resolution is greatest for the acoustic microscope when a very short wavelength is passed through the coupling liquid. This requires a liquid with a low sound velocity, such as liquid helium. An acoustic microscope using liquid helium can resolve surface features as small as 500 Angstroms, but only when the sample is cooled to 0.1 K.
Several additional techniques, not involving generation and detection of stress pulses, are available for measuring film thickness. Ellipsometers direct elliptically polarized light at a film sample and analyze the polarization state of the reflected light to determine film thickness with an accuracy of 3-10 Angstroms. The elliptically polarized light is resolved into two components having separate polarization orientations and a relative phase shift. Changes in polarization state, beam amplitudes, and phase of the two polarization components are observed after reflection.
The ellipsometer technique employs films which are reasonably transparent. Typically, at least 10% of the polarized radiation must pass through the film. The thickness of metal sample films thus cannot exceed a few hundred Angstroms.
Another technique uses a small stylus to mechanically measure film thickness. The stylus is moved across the surface of a substrate and, upon reaching the edge of a sample film, measures the difference in height between the substrate and the film. Accuracies of 10-100 Angstroms can be obtained. This method cannot be used if the film lacks a sharp, distinct edge, or is too soft in consistency to accurately support the stylus.
Another non-destructive method, based on Rutherford Scattering, measures the energy of backscattered helium ions. The lateral resolution of this method is poor.
Yet another technique uses resistance measurements to determine film thickness. For a material of known resistivity, the film thickness is determined by measuring the electrical resistance of the film. For films less than 1000 Angstroms, however, this method is of limited accuracy because the resistivity may be non-uniformly dependent on the film thickness.
In yet another technique, the change in the direction of a reflected light beam off a surface is studied when a stress pulse arrives at the surface. In a particular application, stress pulses are generated by a piezoelectric transducer on one side of a film to be studied. A laser beam focused onto the other side detects the stress pulses after they traverse the sample. This method is useful for film thicknesses greater than 10 microns.
A film may also be examined by striking a surface of the film with an intense optical pump beam to disrupt the film""s surface. Rather than observe propagation of stress pulses, however, this method observes destructive excitation of the surface. The disruption, such as thermal melting, is observed by illuminating the site of impingement of the pump beam with an optical probe beam and measuring changes in intensity of the probe beam. The probe beam""s intensity is altered by such destructive, disruptive effects as boiling of the film""s surface, ejection of molten material, and subsequent cooling of the surface.
See Downer, M. C.; Fork, R. L.; and Shank, C. V., xe2x80x9cImaging with Femtosecond Optical Pulsesxe2x80x9d, Ultrafast Phenomena IV, Ed. D. H. Auston and K. B. Eisenthal (Spinger-Verlag, N.Y. 1984), pp. 106-110.
Other systems measure thickness, composition or concentration of material by measuring absorption of suitably-chosen wavelengths of radiation. This method is generally applicable only if the film is on a transparent substrate.
In a nondestructive ultrasonic technique described in U.S. Pat. No. 4,710,030 (Tauc et al.), a very high frequency sound pulse is generated and detected by means of an ultrafast laser pulse. The sound pulse is used to probe an interface. The ultrasonic frequencies used in this technique typically are less than 1 THz, and the corresponding sonic wavelengths in typical materials are greater than several hundred Angstroms. It is equivalent to refer to the high frequency ultrasonic pulses generated in this technique as coherent longitudinal acoustic phonons.
In more detail, Tauc et al. teach the use of pump and probe beams having durations of 0.01 to 100 psec. These beams may impinge at the same location on a sample""s surface, or the point of impingement of the probe beam may be shifted relative to the point of impingement of the pump beam. In one embodiment the film being measured can be translated in relation to the pump and probe beams. The probe beam may be transmitted or reflected by the sample. In a method taught by Tauc et al. the pump pulse has at least one wavelength for non-destructively generating a stress pulse in the sample. The probe pulse is guided to the sample to intercept the stress pulse, and the method further detects a change in optical constants induced by the stress pulse by measuring an intensity of the probe beam after it intercepts the stress pulse.
In one embodiment a distance between a mirror and a corner cube is varied to vary the delay between the impingement of the pump beam and the probe beam on the sample. In a further embodiment an opto-acoustically inactive film is studied by using an overlying film comprised of an opto-acoustically active medium, such as arsenic telluride. In another embodiment the quality of the bonding between a film and the substrate can be determined from a measurement of the reflection coefficient of the stress pulse at the boundary, and comparing the measured value to a theoretical value.
The methods and apparatus of Tauc et al. are not limited to simple films, but can be extended to obtaining information about layer thicknesses and interfaces in superlattices, multilayer thin-film structures, and other inhomogeneous films. Tauc et al. also provide for scanning the pump and probe beams over an area of the sample, as small as 1 micron by 1 micron, and plotting the change in intensity of the reflected or transmitted probe beam.
While well-suited for use in many measurement applications, it is an object of this invention to extend and enhance the teachings of Tauc et al.
It is thus an object of this invention to provide an improved optical generator and detector of stress pulses.
It is a further object of this invention to provide an improved ultrafast optical technique for measuring stress in a thin film.
It is still another object of this invention to provide an improved ultrafast optical technique for determining the elastic modulus, sound velocity, and refractive index of a thin film.
It is a still further object of this invention to provide an improved ultrafast optical technique for characterizing an interface between two materials, such as an interface between a substrate and an overlying thin film.
It is another object of this invention to provide an ultrafast optical technique for determining a derivative of a transient response of a sample to a pump pulse, and for correlating the derivative with a characteristic of interest, such as the static stress within the sample.
It is another object of this invention to provide an ultrafast optical technique for varying a temperature of the sample and, while varying the temperature, for determining a derivative of the acoustic velocity within the sample and for subsequently correlating the derivative of the acoustic velocity with the static stress within the sample.
It is another object of this invention to provide an ultrafast optical technique for determining an electrical resistivity of a sample.
It is a further object of this invention to provide simulation methods for modelling a time-evolved effect of a stress pulse generated within a sample of interest, and to then employ the model to characterize the sample.
It is a further object of this invention to provide an ultrafast optical technique for measuring a characteristic of interest in a patterned, periodic, multilayered structure.
It is one still further object of this invention to provide an ultrafast optical system and technique wherein optical fibers are used to advantage for directing and/or focussing at least one of an incident pump beam, and incident probe beam, or a reflected or transmitted probe beam.
It is another object of this invention to provide a non-destructive system and method for simultaneously measuring at least two transient responses of a structure to a pump pulse, the measured transient responses comprising at least two of a measurement of a modulated change xcex94R in an intensity of a reflected portion of a probe pulse, a change xcex94T in an intensity of a transmitted portion of the probe pulse, a change xcex94P in a polarization of the reflected probe pulse, a change xcex94"PHgr" in an optical phase of the reflected probe pulse, and a change in an angle of reflection xcex94xcex2 of the probe pulse.
It is one further object of this invention to provide a non-destructive system and method for determining a characteristic of a sample that includes an automatic control over the focussing of pump and probe beams at the sample so as to provide a reproducible intensity variation of the beams during each measurement.
The foregoing and other problems are overcome and the objects of the invention are realized by methods and apparatus in accordance with embodiments of this invention.
This invention relates to a system for the characterization of thin films and interfaces between thin films through measurements of their mechanical, optical, and thermal properties. In the system of this invention incident light is absorbed in a thin film or in a structure made up of several thin films, and the change in optical transmission or reflection is measured and analyzed. The change in reflection or transmission is used to give information about the ultrasonic waves that are produced in the structure. The information that is obtained from the use of the measurement methods and apparatus of this invention can include: (a) a determination of the thickness of thin films with a speed and accuracy that is improved compared to earlier methods; (b) a determination of the thermal, elastic, electrical, and optical properties of thin films; (c) a determination of the stress in thin films; and (d) a characterization of the properties of interfaces, including the presence of roughness and defects.
The invention features a radiation source for providing a pump beam and a detection system for non-destructively measuring the properties of one or more interfaces within a sample. The radiation source provides the pump beam so as to have short duration radiation pulses having an intensity and at least one wavelength selected to non-destructively induce a propagating stress wave in the sample, a radiation source for providing a probe beam, a mechanism for directing the pump beam to the sample to generate the stress wave within the sample, and a mechanism for guiding the probe beam to a location at the sample to intercept the stress wave. A suitable optical detector is provided that is responsive to a reflected or transmitted portion of the probe beam for detecting a change in the optical constants of the material induced by the stress wave.
In one embodiment, the optical detector measures the intensity of the reflected or transmitted probe beam. The pump and probe beam may be derived from the same source that generates a plurality of short duration pulses, and the system further includes a beam splitter for directing a first portion of the source beam to form the pump beam, having the plurality of pulses, and directing a second portion to form the probe beam, also having the plurality of pulses. The source beam has a single direction of polarization and the system further includes means for rotating the polarization of the probe beam and a device, disposed between a sample and the optical detector, for transmitting only radiation having the rotated direction of polarization. The system may further include a temperature detector and a chopper for modulating the pump beam at a predetermined frequency. The system can further include a mechanism for establishing a predetermined time delay between the impingement of a pulse of the pump beam and a pulse of the probe beam upon the sample. The system can further include circuitry for averaging the output of the optical detector for a plurality of pulse detections while the delay between impingements remains set at the predetermined time delay. The delay setting mechanism may sequentially change the predetermined time delay and the circuitry for averaging may successively average the output of the optical detector during each successive predetermined time delay setting.
By example, the pump beam may receive 1% to 99% of the source beam, and the source beam may have an average power of 10 xcexcW to 10 kW. The source beam may include wavelengths from 100 Angstroms to 100 microns, and the radiation pulses of the source beam may have a duration of 0.01 psec to 100 psec.
The sample may include a substrate and at least one thin film to be examined disposed on the substrate such that interfaces exist where the films meet, and/or where the film and the substrate meet. For a sample with an optically opaque substrate, at the pump wavelength, the pump and probe beams may both impinge from the film side, or the pump may impinge from the film side and the probe may impinge from the substrate side. For a sample with a transparent substrate, both beams may impinge from the film side, or from the substrate side, or from opposite sides of the sample. The optical and thermal properties are such that the pump pulse changes the temperature within at least one film with respect to the substrate. The temperature within one or more of the thin films disposed on the substrate may be uniform, and may be equal in several films. The films may have thicknesses ranging from 1 xc3x85 to 100 microns. At least one film in the sample and/or the substrate has the property that when a stress wave is present it causes a change in the intensity, optical phase, polarization state, position, or direction of the probe beam at the detector. The probe beam source may provide a continuous radiation beam, and the pump beam source may provide at least one discrete pump pulse having a duration of 0.01 to 100 psec and an average power of 10 xcexcW to 1 kW. Alternatively the probe beam source may provide probe beam pulses having a duration of 0.01 to 100 psec, the pump beam and probe beam may impinge at the same location on the sample, and the mechanisms for directing and guiding may include a common lens system for focusing the pump beam and the probe beam onto the sample. The position of impingement of the probe beam may be shifted spatially relative to that of the pump beam, and the probe beam may be transmitted or reflected by the sample.
One or more fiber optic elements may be incorporated within the system. Such fibers may used to guide one or more beams within the system for reducing the size of the system, and/or to achieve a desired optical effect such as focussing of one or more beams onto the surface of the sample. To achieve focussing, the fiber may be tapered, or may incorporate a small lens at its output. A similar focussing fiber can be used to gather reflected probe light and direct it to an optical detector. A fiber may also be used to modify the beam profile, or as a spatial filter to effect a constant beam profile under widely varying input beam conditions.
This invention advantageously provides a non-destructive system and method for measuring at least one transient response of a structure to a pump pulse of optical radiation, the measured transient response or responses including at least one of a measurement of a modulated change xcex94R in an intensity of a reflected portion of a probe pulse, a change xcex94T in an intensity of a transmitted portion of the probe pulse, a change xcex94P in a polarization of the reflected probe pulse, a change xcex94"PHgr" in an optical phase of the reflected probe pulse, and a change in an angle of reflection A6 of the probe pulse, each of which may be considered as a change in a characteristic of a reflected or transmitted portion of the probe pulse. The measured transient response or responses are then associated with at least one characteristic of interest of the structure.
In a presently preferred embodiment the system provides for automatically focusing the pump and probe pulses to achieve predetermined focusing conditions, and the application of at least one calibration factor to the at least one transient response. This embodiment is especially useful when employed with time-evolved simulations and models of a structure of interest, which is a further aspect of this invention.